Most Heighten effects are purely beneficial, but some significantly alter the effect of a spell. There is an edge case where you may want to use a high level spell slot but get the effect unaltered. The most obvious instance of this, to me, is Invisibility
Duration 10 minutes
Cloaked in illusion, the target becomes invisible. This makes it undetected to all creatures, though the creatures can attempt to find the target, making it hidden to them instead. If the target uses a hostile action, the spell ends after that hostile action is completed.
Heightened (4th) The spell lasts 1 minute, but it doesn’t end if the target uses a hostile action.
A hypothetical, to illustrate, would be attempting to bypass a Dispel trap or otherwise expecting the Invisibility to be Counteracted while still wanting the 10 minute duration. Could I cast Invisibility using a 4th+ spell slot and still maintain the normal Duration and benefits/limitation of the spell?